The invention relates to a method of photolithographically metallizing at least the inside of holes arranged in accordance with a pattern in a plate of an electrically insulating material, while using a negative, photosensitive cataphoretic lacquer coating.
Such perforated plates, for example, of glass, quartz glass, synthetic resin or a ceramic material are used, in particular, in field-emission displays and thin electron displays, the metallized holes serving as electrodes to control the electron currents moving towards the luminescent screen provided with phosphors, and the electron currents in plasma displays. The surface of such a plate also comprises narrow metal tracks which are used to drive (address) the electrodes. In such displays, the plates are used as control or selection elements. Thin electron displays comprise at least two selection plates.
Such a thin electron display is described in European Patent Application EP-A-464937, filed by Applicants. The display described therein comprises, inter alia, a preselection plate and a fine selection plate. These plates are, for example, made of glass and have a thickness of 0.5 mm. Such a plate comprises a very accurate pattern of holes, for example 400,000 holes each having a diameter of 400 .mu.m. In and around said holes there are provided metal selection electrodes which can be individually activated by means of narrow metal tracks on the glass plate material. In said Patent Application no information is given as to which metal is used as the electrode nor how the metal is provided in and around the holes.
In European Patent Application EP-A-539714, there is disclosed an electrophoretic method of manufacturing copper patterns on printed circuit boards by using a negative, photosensitive cataphoretic lacquer. A substrate, for example, of epoxy to which a copper layer is uniformly applied is immersed in a solution of such a lacquer, the substrate and an inert electrode, for example, of stainless steel being connected to an external current source. The copper layer on the substrate serves as the cathode and the inert electrode as the anode, i.e. the copper layer is electrically connected to the negative pole of the current source. The solution of the lacquer comprises, inter alia, a mixture of a polymer comprising positively charged groups, unsaturated monomers and a photoinitiator. Examples of positively charged groups are the amino group, quaternary ammonium group, sulphonium group and sulphoxonium group. The electric field in the solution causes the positively charged mixture to be attracted towards the negatively charged copper layer where it is discharged. This results in the formation of a dense and uniform lacquer coating on the copper layer. Such a cataphoretic lacquer must be used to provide three-dimensional structures, such as internally metallized holes. The photosensitive lacquer is of the negative type, i.e. exposed parts of the lacquer coating become less soluble in a developing liquid than unexposed parts as a result of a photochemical reaction. Exposure often takes place through a photomask. This difference in solubility enables the lacquer coating to be selectively removed. The lacquer pattern formed serves as a mask for an etching agent, so that the copper layer is structured to form a desired pattern which corresponds to the photomask used.
A disadvantage of this state-of-the-art method is that copper cannot be used as a metal track or electrode in thin electron displays because of undesirable interactions with the electroluminescent phosphors used.
The metal tracks of the selection plates, for example, have a width of only 80 .mu.m and a thickness of 5 .mu.m. Under operating conditions, metal tracks having such a small cross-section lead to an enormous ohmic drop, over, a distance of 40 cm if an electrically ill-conducting metal is used. Examples of electrically well-conducting metals are copper and silver. Copper is unsuitable because of the above-mentioned disadvantage; silver is unattractive because it is expensive. It should be borne in mind that in order to obtain the desired pattern, the major part of the metal layer is removed by means of an etchant.
As aluminum exhibits a good electrical conductivity and a high vapour-deposition or sputtering rate, it can be used as the metal for, in particular, the metal tracks and the electrodes. It has however been found that aluminum is attacked in the known solutions of cataphoretic lacquers. If aluminum is brought into contact with a solution of a cataphoretic lacquer, gas bubbles form at the surface of the aluminum and the aluminum is dissolved in the lacquer solution. As a result, the cataphoretic lacquer coating is deposited in such a large thickness that the light used in the photolithographic step is absorbed to a substantial degree. Exposure of the lacquer coating throughout its thickness and complete curing then becomes problematic, resulting in a poor-quality photolithographic process. In addition, when the lacquer coating is provided, the gas bubbles partially dissolve in the lacquer coating, thereby forming undesirable pinholes in the lacquer coating.